Rare earth doped amplifiers, such as erbium doped fiber amplifiers (EDFA), are widely used in optical communication networks. Their gain is a function of the wavelength of the signal, which is temperature sensitive. As these networks evolved to several channels and are typically deployed in operating environments with changing temperatures, their thermal spectral dependence becomes a matter of concern. The solution of operating in temperature-controlled environment is energy consuming. An easier solution is to operate the device at a constant hot temperature, but the aging problems must then be faced. Filters have been proposed to compensate for the thermal spectral dependence. For example, U.S. Pat. No. 6,049,414 describes a design comprising a plurality of concatenated filters having center wavelengths which spectrally shift with temperature to compensate temperature-dependent shifts in the gain of the amplifier. This U.S. patent deals with a composite loss filter based on long period fiber gratings that maintains desired flatness of an EDFA even as the operating temperature changes. While this design appears to achieve the claimed goal of temperature compensation for the drift of the amplifier, it is complicated and requires the presence of a variable attenuator.
U.S. Pat. No. 5,042,898 discloses a temperature compensated Bragg filter, which is again a filter with embedded gratings. In this case, temperature compensation is achieved by mounting the filter on two compensating members, each made of a different material. One of the materials can be aluminum and the other stainless steel. These materials have such coefficients of thermal expansion relative to one another and to that of the fiber material as to apply to the fiber longitudinal strains, the magnitude of which varies with temperature in such a manner that the changes in the longitudinal strains compensate for those attributable to changes in temperature of the grating. This again is a fairly complex arrangement requiring juxtaposition of the two compensating members, with pre-loading features that can loosen or tighten the arrangement to the extent needed for the grating region to be reflective to light in the narrow region around the desired wavelength.
In the article entitled “Passive temperature-compensating package for optical fiber gratings” by G. W. Yoff et al., published in Applied Optics, Vol. 34, No. 30, Oct. 20, 1995, the problem of passive temperature compensation for optical fiber gratings is again addressed. The authors use a compact package also comprising two materials with different coefficients of thermal-expansion. The idea expressed in this article is to choose one material with a low thermal-expansion coefficient α, such as silica (α=5×10−7/° C.) and another material with a high thermal expansion coefficient, such as aluminum (α=2.5×10−5/° C.) or stainless steel (α=1.7×10−5/° C.), rather than two materials with a small difference between the expansion coefficients. Apart from providing a more compact device, the filter of this article is still a complex Bragg filter based on gratings and requires mounting of the two materials with different thermal-expansion coefficients with adjustable tension relative to each other.
All the above discussed filters are based on gratings imprinted in the core of the fiber and forming discontinuities so that when the light is launched into the fiber core for guided propagation, only that having a wavelength within a narrow range can pass in the propagation direction. This is quite different from the tapered fiber filters of the present invention, although, as stated in applicant's International PCT application WO 01/02886, the basic principle of compensating the temperature dependent optical effect applies equally to Bragg gratings and to tapered fiber filters. In this International application, applicant has disclosed a combination of an optical component, such as a tapered fiber filter, being solidly secured to a rigid substrate that produces a mechanical stress to effect elongation of the component so as to compensate for any modal phase shift due to temperature variation. Also, the mechanical phase dependence of the component may be adjusted in relation to the substrate to provide the desired temperature compensating effect. The substrates used for the above purpose were special types of silica glass having thermal expansion coefficients greater than quartz. Such devices are, however, not suitable for very strong temperature dependence that is normally required in EDFA.